1. Field of the Invention
The present invention relates to an image forming apparatus including a developing unit using a two-ingredient type developer, i.e., a toner and carrier mixture and more particularly to a developer replenishing device for replenishing the developer to the developing unit.
2. Description of the Related Art
A printer, copier, facsimile apparatus, multifunction machine or similar image forming apparatus of the type using a developing unit that develops a latent image with a two-ingredient type developer or toner and carrier mixture is conventional. It is a common practice with this type of image forming apparatus to replenish fresh toner from a toner cartridge or similar toner container and replace, when the toner cartridge runs out of toner, the toner container with a new toner container.
Although the carrier is not consumed by image formation, it is deteriorated in ability due to repeated image formation and must therefore be replaced every time image formation is repeated a certain number of times. Generally, a service person replaces the carrier in the case of a copier or bodily replaces, in the case of a printer, the entire developing unit in which the carrier is present.
On the other hand, Japanese Patent Laid-Open Publication No. 11-212346, for example, proposes to replenish a fresh carrier from a carrier container to a developing unit little by little. Also, Japanese Patent Laid-Open Publication No. 2001-209244 teaches a system in which a fresh carrier is caused to join fresh toner being replenished while excess carrier is discharged from a developing unit little by little. This system, according to the above document, obviates the deterioration of a developer for thereby implementing stable image quality over a long period of time without resorting to periodic carrier replacement.
However, many of conventional carrier replenishing devices, including the schemes described above, use a screw for conveying the carrier, so that a carrier conveying path must be arranged substantially straight, i.e., cannot be extended upward. This limitation, combined with other limitations ascribable to the screw, causes the carrier storing portion of the carrier replenishing device to be necessarily positioned in the vicinity of a developing unit. Such a position is undesirable because a toner and a carrier replenishing device cannot be located at a position where they can be easily replaced by the user of the apparatus and because size reduction of the apparatus is obstructed. Moreover, in the case where the carrier is replenished independently of the toner, an exclusive space must be allocated to the carrier replenishing device, increasing the overall size of the apparatus. This is particularly true in a color image forming apparatus needing a plurality of developer replenishing devices.
In light of the above, there has been proposed a developer replenishing device of the type using a powder pump for conveying a developer. This type of developer replenishing device conveys a toner and carrier mixture via a hollow tube with the pressure of the powder pump and compressed air that fluidizes the mixture, thereby implementing noticeably free layout of a conveying path. This, in turn, promotes the free layout of a toner and a carrier storing section and that of a developing unit for thereby facilitating the replacement of a toner and a carrier cartridge and implementing size reduction of the apparatus. However, this type of developer replenishing device is not simple in construction and occupies a large exclusive space in the case of a color image forming apparatus needing a plurality of developer replenishing devices, increasing the overall size of the apparatus.